Study On The Evaluation Method Of Tight Sandstone Fluid State And Irreducible Water Saturation

With the large-scale exploration and development of unconventional oil and gas resources,the study of tight sandstone has been paid more and more attention.The characterization of fluid state and the quantitative evaluation method of bound fluid saturation play an important role in tight sandstone oil and gas reservoir evaluation,productivity prediction and oil and gas recovery.Tight sandstones have abundant geological genetic types and varied mineral types.There are many micro and nano pores in rocks,and the pore structure is complex,which makes the response characteristics of the conventional petrophysical experiment methods not obvious,and it is difficult to obtain effective information.Nuclear magnetic resonance(NMR)method can directly detect the hydrogen nuclei in the pore media to obtain the fluid properties in the pores of the rock,which plays a crucial role in the evaluation of tight sandstone.In this paper,based on the quantitative mineral analysis of digital rocks and high precision micron CT,the mineral skeleton model and pore system model of tight sandstone rocks are established.By analyzing the state of fluid molecules in the pore system in the above skeleton model,the fluid NMR relaxation mechanism in the pores of tight sandstone is established.Based on the NMR relaxation mechanism in the tight sandstone,the intermolecular interactions between hydrogen nuclei with rotation constraints(isolated spin pairs)obtained by solid echo and regulating bound fluid by BE-CPMG are used to measure T₂ and obtain different information from spin echo T₂,so as to study the fluid state and bound water saturation.At last,the results of T₂ and T₁-T₂ solid echo responses in tight sandstone are used to show the fluid state of single clay peak signal in high clay content pores and the signal of clay merging with pore fluid in low clay content pores.The quantitative calculation of irreducible water saturation evaluation method is proposed by BE-CPMG to control bound fluid in tight sandstone.